In recent years, passive devices, such as a silicon-based waveguide, an optical coupler, and a wavelength filter are researched very widely. Moreover, as an important technology for manipulating an optical signal for a communication system, active elements, such as a silicon-based optical modulator and an optical switch are attracting significant attention. Here, an optical switch and a modulation element, which use the thermo-optic effect in silicon to change a refractive index has a low speed, and can be used only with a modulation frequency (device speed) of about 1 Mb/sec. Therefore, in order to achieve a high modulation frequency required in many optical communication systems, an optical modulation element using an electro-optic effect is required.
One of electro-optic devices is a silicon-based optical communication device. The silicon-based optical communication device can be used with light having wavelengths of 1,310 nm and 1,550 nm, which are used in optical fibers for various systems, such as fiber-to-the-home and a local area network (LAN). Moreover, the silicon-based optical communication device can be realized, using the complementary metal oxide semiconductor (CMOS) technology, as an integrated circuit obtained by integrating optical functional elements and electronic circuits on a silicon platform. Therefore, the silicon-based optical communication device is a technology that is very promising for realizing an optical modulation element having a high modulation frequency.
Most related electro-optic devices (optical modulators) that have currently been proposed changes a free carrier density in a silicon layer using the carrier plasma effect to change a real part and an imaginary part of the refractive index, to thereby change a phase and an intensity of light. Here, pure silicon does not exhibit the linear electro-optic effect (Pockels effect). Moreover, a change in refractive index caused by the Franz-Keldysh effect and the Kerr effect in pure silicon is very small. Therefore, in the related electro-optic devices, the above-mentioned carrier plasma effect is widely used. Moreover, a modulator using free carrier absorption directly modulates output by changing absorption of light that propagates through silicon (Si). As the structure using a change in refractive index, the structure using a Mach-Zehnder interferometer is generally used. This structure generates a phase difference between optical signals passing through two arms, and multiplexes those optical signals to obtain a light intensity modulation signal.
The free carrier density in the electro-optic device can be changed by injecting, accumulating, removing, or inverting free carriers. However, most of such related devices that have currently been investigated have low optical modulation efficiency. Therefore, those related devices require a length on the order of mm for optical phase modulation, and require an injection current density of more than 1 kA/cm3. Moreover, as an element size becomes larger, the electro-optic device becomes more likely to be affected by a temperature distribution on the silicon platform. Therefore, in the electro-optic device having a large size, a refractive index of the silicon layer is changed by the thermo-optic effect. As a result, with an optical phase modulator having low efficiency, there is assumed a situation in which the original electro-optic effect is canceled. Therefore, in order to achieve downsizing, higher integration, and lower power consumption, there is a need for the element structure with which high optical modulation efficiency can be obtained. The high optical modulation efficiency can be achieved to reduce an optical phase modulation length. In order to achieve the high optical modulation efficiency, it is desired that a region in which the carrier density is changed and controlled ideally be the same as an optical signal field.
FIG. 1 is a view for illustrating a typical example of a silicon-based electro-optic phase modulator using the rib waveguide structure formed on a silicon-on-insulator (SOI) substrate, which is described in Non Patent Document 1. In this electro-optic phase modulator, slab regions 102 and 103 extending transversely on both sides of a rib shape formed of an intrinsic semiconductor region 101 are formed by being p-doped and n-doped, respectively. The above-mentioned rib waveguide structure is formed using an intrinsic semiconductor silicon layer 1 on the SOI substrate. The modulator illustrated in FIG. 1 is a positive intrinsic-negative (PIN) diode type modulator. This modulator has the structure in which a forward bias or a reverse bias is applied to change a free carrier density in the intrinsic semiconductor region 101 and hence generate the carrier plasma effect, to thereby change a refractive index of the region 101. In this example, the SOI substrate includes a support substrate 3, a buried oxide film 2 formed on the support substrate 3, and the intrinsic semiconductor silicon layer 1 formed on the buried oxide film 2. The intrinsic semiconductor silicon layer 1 includes a p-type region (p+-doped semiconductor silicon layer) 4, which is formed by doping at a high concentration, and a part of which forms a first electrical contact portion 6-1. In FIG. 1, the intrinsic semiconductor silicon layer 1 further includes an n-type region (n+-doped semiconductor silicon layer) 5, which is formed by n-type doping at a high concentration. A part of the p-type region 4 serves as the first electrical contact portion 6-1, and a part of the n-type region 5 serves as a second electrical contact portion 6-2. Electrode wirings 7-1 and 7-2 are connected to the electrical contact portions 6-1 and 6-2, respectively. The electrode wirings 7-1 and 7-2 are formed to pierce an oxide cladding 8, which is formed to cover the intrinsic semiconductor silicon layer 1. In the PIN diode structure, the p- and n-type regions 4 and 5 may be doped to exhibit a carrier density of about 1,020 per cm3. Moreover, in the PIN structure, the p-type region 4 and the n-type region 5 are spaced apart on both sides of the intrinsic semiconductor silicon layer 1 having a rib shape.
When the forward bias is applied to the PIN diode through the first and second electrical contact portions 6-1 and 6-2, free carriers are injected into the waveguide. At this time, with the increase of the free carriers, a refractive index of the intrinsic semiconductor silicon layer 1 is changed. As a result, light transmitted through the waveguide is phase-modulated. However, the speed of this optical modulation operation is limited by free carrier lifetime in the intrinsic semiconductor silicon layer 1 having the rib shape and a carrier diffusion rate after the forward bias is removed. Such PIN diode phase modulator generally has an operation speed in a range of from 10 Mb/sec to 50 Mb/sec during the forward bias operation. In contrast, an operation speed after the forward bias is removed is extremely low. Impurities may be introduced into the silicon layer to shorten the carrier lifetime, to thereby increase a switching speed. However, the introduced impurities reduce the optical modulation efficiency. In addition, the most significant factor affecting the operation speed is the RC time constant. In this PIN diode phase modulator, a capacitance (C) at the time when the forward bias is applied is very large because of a reduction of a carrier depletion layer of a PN junction. Theoretically, it is possible to achieve a high-speed operation by applying the reverse bias to the PN junction. However, a relatively large drive voltage or a large element size is required therefor.
Incidentally, in Patent Document 1, there is described a silicon-based electro-optic modulator, which is formed of a body region of a second conductivity type, and a gate region of a first conductivity type, which is laminated to partially overlie the body region, and in which a relatively thin dielectric layer is formed at an interface of the lamination. Also in Patent Document 2, an SOI-based optical arrangement having the similar structure is described.
FIG. 2 is a view for illustrating a silicon-based electro-optic modulator having a silicon-insulator-silicon (SIS) type structure of the above-mentioned type. In FIG. 2, constituent elements equivalent to the constituent elements of FIG. 1 are denoted by the same reference numerals.
The silicon-based electro-optic modulator of FIG. 2 is formed on an SOI platform. A body region 105 of the silicon-based electro-optic modulator is formed in an intrinsic semiconductor silicon layer 106 of the SOI substrate. Moreover, a gate region 107 is formed in a polysilicon layer 108, which is laminated to form the SOI structure. A dielectric layer 12 is arranged between the body region 105 and the gate region 107. The silicon layer 106 and the polysilicon layer 108 are doped to form p-doped semiconductor silicon (first silicon semiconductor layer 9) and n-doped polysilicon (second silicon semiconductor layer 10), respectively. Moreover, the silicon layer 106 and the polysilicon layer 108 includes the p-type region 4 and an n-type region (n+-doped polysilicon layer) 11, which are doped at a high concentration, respectively. A part of the first silicon semiconductor layer 9 and a part of the second silicon semiconductor layer 10 serve as the body region 105 and the gate region 107, respectively. In those doped regions, the carrier density is changed by an external signal voltage. In other words, the carrier density of the doped regions can be dynamically and externally controlled. On both sides of the dielectric layer 12, free carriers are accumulated, removed, or inverted to phase-modulate the light passing through the body region 105. In order to efficiently achieve the phase modulation, it is desired that the region in which the carrier density is dynamically and externally controlled ideally be the same as the optical signal field. However, in reality, the region in which the carrier density is dynamically changed has very small thickness of about several ten nm as compared to the spread of the optical signal field. Therefore, in order to achieve desired optical phase modulation, an optical modulation length (length in the front-and-back direction of FIG. 2) on the order of mm is required. As a result, the size of the electro-optic device is increased, and the high-speed operation becomes difficult.
Moreover, in Patent Document 3, a silicon-based electro-optic device (metal oxide semiconductor (MOS) type Si modulator) is described. In this electro-optic device, a laminate structure of a first silicon semiconductor layer of a first conductivity type and a second silicon semiconductor layer of a second conductivity type has a rib waveguide shape to form an optical confinement region. Moreover, as illustrated in FIG. 3, this electro-optic device includes, in a slab part 111 of a rib-type waveguide 110, a region to which the electrode wiring (metal electrode) 7-1 is connected. In this region to which the electrode wiring 7-1 is connected, a thickness of the slab part 111 is larger than a thickness of the surrounding slab part 111. Further, the regions to which the electrode wirings 7-1 and 7-2 are connected are formed in a range of thickness in which, when a distance from the regions to the rib-type waveguide 110 is changed, an effective refractive index of the rib-type waveguide 110 in a zeroth-order mode does not change.
In this silicon-based electro-optic device, it needs to be ensured that interference does not occur between an electrode layer (not shown) electrically connected to the first silicon semiconductor layer 9 of the first conductivity type and an electrode layer (not shown) electrically connected to the second silicon semiconductor layer 10 of the second conductivity type. In other words, the electrode layer electrically connected to the first silicon semiconductor layer 9 needs to be formed at a position that is spaced apart from the electrode layer electrically connected to the second silicon semiconductor layer 10. Here, when the electrode layer electrically connected to the second silicon semiconductor layer 10 of the second conductivity type is arranged near the rib-type waveguide 110 (center side in FIG. 3), an optical loss is increased. Therefore, the electrode layer electrically connected to the second silicon semiconductor layer 10 of the second conductivity type needs to be spaced apart from the rib-type waveguide 110 to a certain extent. Therefore, the electrode layer connected to the first silicon semiconductor layer 9 needs to be formed at a position that is spaced further away from the rib-type waveguide 110. As a result, a wiring distance from the first silicon semiconductor layer 9 to the electrode layer is increased, and an extraction resistance is increased.